The invention relates to the field of electrical power conversion and regulation systems.
Various types of power conversion and regulation systems are well-known in the art. Modern electronic equipment which use microelectronic circuits or transistors generally require a low-voltage high-current power supply. Especially in devices which use digital logic, such as computers or microprocessors, there is a need to provide high power levels at a very precise voltage since in many such systems the difference in logic states is less than one volt.
Present techniques for regulating power involve the use of either dissipative regulators or switch mode regulators. Linear power supplies and associated dissipated regulators are highly inefficient and wasteful of energy and require bulky magnetics and massive heatsinks.
The basic techniques of switching power supplies are well-known, as shown in the article "Focus on Switching Power Supplies", Electronic Design, Vol. 26, No. 17, August 16, 1978, pp. 72-83. It is known that switching power supplies are highly efficient. In DC to DC converters, a switching power supply can deliver 60-90% efficiency as compared to linear supplies ' 20-50% efficiences. Since most switching power supplies operate at around 20 kHz, components can be made smaller and lighter than in linear supplies.
However, prior art switching supplies suffer from several disadvantages. Chief among them that power regulation is generally poorer than with linear supplies. Also, prior art switching supplies produce a considerable amount of electromagnetic interference due to approximately 50 mv peak-to-peak noise pulses. Prior art switching supplies are limited in their power handling capabilities since the components capable of handling high power levels are intrinsically "slow" (i.e. have a long recovery period between power saturation cycles). This limits the use of high power components to lower switching frequencies (e.g. below 20 KHz) which lower the overall efficiency of the system and results in greater component size and high levels of acoustic noise. Typically rise and fall times of high power switching SCRs are limited to .apprxeq.10 .mu.sec or more which limits their dynamic range when operated at pulse width modulation (switching) frequencies above the audio range (20 KHz). Of course, switching transistors could be paralleled for high power handling; however, the matching and balancing requirements for a high power system renders this solution impractical.
Prior art switching supplies such as that shown in the Electronic Design article at page 78 comprise a DC input applied to a switching circuit (typically a number of SCRs driven at 20 KHz) with the high frequency square wave output of the switching circuit being applied to a voltage conversion device such as an inductor or transformer. The output of the transformer is then rectified and filtered to produce pure DC without a high frequency ripple. In order to regulate the output voltage, a portion of the DC output is sampled and compared with a reference voltage to produce an error signal which controls a pulse modulation circuit. The modulation circuit controls the firing of the SCR switching transistors to produce a regulated DC output.
U.S. Pat. Nos. 3,416,062 and 3,205,424 are representative of prior art pulse width modulated (switching) power supplies having a feedback loop from the output to the switching oscillator. The pulse train generated by the oscillator generally comprises a series of square waves accompanied by spurious harmonics. These harmonics are eliminated by a band pass filter placed in the output of the oscillator.
U.S. Pat. No. 4,042,873 discloses a phase-locked loop voltage regulator having a voltage controlled oscillator (VCO) which controls the firing of an SCR pulse driving circuit through a ramp generator and comparator circuit. An AC input is applied to a phase detector which compares the phase angle of the applied power with the phase angle of the VCO output. A detected phase difference is filtered through a low pass filter and used to control the VCO. A separate comparator circuit detects the voltage output of the VCO and compares it with a reference voltage to control the conduction of the VCO output to the SCR pulse driving circuit.
It is known to employ an active harmonic filter in a Class C inverter circuit. In a Class C inverter pure DC is converted to pulsed DC by a capacitor or LC network switched by a load-carrying SCR. An active harmonic filter, such as that described in the GE SCR Manual, 4th ed, 1967, pp. 235-236 and 248, has a frequency response such that for a fixed input voltage the output voltage is a function of the input frequency. Such a filter is also known as an OTT filter, after its inventor. However, in the circuit described the OTT filter merely acts as a harmonic filter to provide a sine wave output to a load. The filter provides good load regulation while maintaining a capacitive load to the Class C inverter over a large range of load power factors. It will be noted however, that the described OTT filter is employed merely as an active harmonic filter and load regulator and plays no part in the voltate regulation scheme of its associated Class C inverter. Indeed, it is noted on p. 226 of the GE SCR Manual that due to design and component limitations a Class C inverter is only useful for switching frequencies below 1000 Hz.
It can thus be seen that there is a need for a simple, efficient and economical power conversion and regulation system which can employ high power and high frequency switching techniques with good output regulation and low noise.